1. Field of the Invention
The present invention relates to improved sealing apparatus and method for sealing an injection unit and a sprue bushing in a coinjection molding machine, and in particular to such apparatus and method where the hot runner assembly and mold are movable with respect to stationary sections of the mold, wherein the interface of at least one coinjection nozzle with at least one hot runner sprue bushing remains sealed throughout the molding cycle (which includes relative movement of the hot runner system). Preferably, the coinjection molding machine comprises a second injection unit mounted adjacent the movable mold section and travels therewith.
2. Description of Related Art
Coinjection molding is typically used to mold multi-layered plastic packaging articles having a laminated wall structure. Each layer is typically passed through a different annular or circular passageway in a single nozzle structure and each layer is partially, sequentially, injected through the same gate. Some coinjection hot runner systems include shooting pots to meter material of one plastic resin so that each cavity of a multi-cavity mold receives an accurate dose of that resin in the molding cycle. In configurations where the melt distribution (hot runner manifold) is mounted in a moving section of the mold it is desirable to maintain this connection.
U.S. Pat. No. 4,080,147 to Dumortier discloses a hot runner manifold mounted in a moving section of the mold. A shooting pot is associated with each nozzle/cavity combination. The shooting pot is charged while the mold is open (FIG. 5), the action causes the hot runner and mold assembly to move away from the manifold backing plate a distance “b”. The mold is closed in FIG. 6, the action causes a valve to block the melt channel to prevent backflow of the resin toward the feeding unit. Then the distance “b” is closed up by the continuing motion of the machine clamp that causes the resin in the shooting pot to be injected into the mold cavity partially filling it. However, there is no teaching of how the injection unit connects to the sprue bushing of the hot runner system, or whether this connection is intermittent or constant.
U.S. Pat. No. Re. 35,256 to von Buren discloses a clamping piston acting perpendicular to the injection unit's axis to intermittently effect a sealing connection to the hot runner system of the mold. The connection is released and sealed during each molding cycle.
U.S. Pat. No. 5,044,927 to DiSimone discloses a latching mechanism for clamping the injection unit's nozzle to the bushing at the end of sprue bar of a stack mold in order to alleviate unequal clamping. The connection is released and sealed during each molding cycle.
U.S. Pat. No. 4,207,051 to Wright and U.S. Pat. No. 5,910,327 to Schad both disclose a stack mold having a telescoping sprue bar that adjusts its length as the mold opens and closes. However the sprue bar's connection with the injection unit's nozzle is released and sealed during each molding cycle.
U.S. Pat. No. 6,413,076 to Dray discloses a machine nozzle that continuously engages the mold's sprue bushing throughout the molding cycle. An extension from the nozzle remains within a corresponding bore extending from the sprue bushing allowing relative axial movement between the two without separation. The channel through the extension and sprue is open only when the injection unit is advanced toward the mold and its transverse channel at one end is aligned with a local widening of the channel in the sprue bushing; consequently it can only provide communication for the melt in this single position. There is no disclosure of communication for the melt in a range of positions in which the nozzle is engaged within the sprue.
WO 02/058908 A1 to Clarke discloses a multi-cavity mold employing a hot runner having shooting pots. The shooting pots are charged while the mold is open without any corresponding movement of the hot runner or cavity plate assembly of the mold. The shooting pots are discharged by the action of closing the mold by the machine clamp, the resin being discharged to completely fill the cavities as the cavity itself closes. The shooting pot piston extends from the cavity side of the mold and is directly acted on by the core plate of the mold during closing. Valves included in the hot runner ensure backflow of the resin toward the feeding unit is prevented during injection.
U.S. Pat. No. 6,152,721 to Schad discloses a shooting pot actuation device that comprises a movable plate mounted behind the stationary platen of the machine having actuation rods extending through the platen to act on the shooting pot pistons mounted in the hot runner of the mold. The hot runner assembly is also mounted to the stationary platen of the mold so that the shooting pot piston actuators never separate contact with their corresponding rod actuators mounted on the movable plate.
U.S. Pat. No. 4,966,545 to Brown discloses a shooting pot piston moved in two discrete motions in order to deliver two metered amounts of resin to the same mold cavity from the same shooting pot. The piston is actuated by tandem mounted cylinders in the mold plate, and valves are used to prevent backflow to the feeding unit during injection. See also:
U.S. patent application Ser. No. 10/879,576 entitled INJECTION MOLDING MACHINE SHOOTING POT WITH INTEGRAL CHECK VALVE;
U.S. patent application Ser. No. 10/879,581 entitled INJECTION MOLDING MACHINE SPIGOTTED SHOOTING POT PISTON;
U.S. patent application Ser. No. 10/879,575 entitled APPARATUS AND METHOD FOR ACTUATION OF INJECTION MOLDING SHOOTING POTS;
U.S. patent application Ser. No. 10/879,582 entitled CONTROL SYSTEM FOR A DYNAMIC FEED COINJECTION PROCESS;
U.S. patent application Ser. No. 10/880,494 entitled HOT RUNNER COINJECTION NOZZLE WITH THERMALLY SEPARATED MELT CHANNELS;
U.S. patent application Ser. No. 10/880,493 entitled COINJECTION MOLDING COOLED SHOOTING POT CYLINDER; and
U.S. patent application Ser. No. 10/887,353 entitled APPARATUS AND METHOD FOR INJECTION MOLDING SHOOTING POT WEDGE FEATURE.
Thus, what is needed is an injection molding machine wherein the hot runner system is disposed in a relatively movable section of the mold, and wherein the interface of the nozzle and the sprue bushing remains sealed throughout the molding cycle.